Many conventional devices are known for guiding and positioning a tool or an element, such as a parts gripper, with respect to a work piece. These devices range from simple hand-operated mechanical devices to more accurate and automatic, fluid operated devices in which the tool can be located in numerous positions by controlling the pressure and amount of the fluid. Such devices are commonly used in a variety of environments to perform a multitude of work functions such as the pick-up placement of parts in assembly lines, and the positioning of work pieces or tools for operations such as punching, drilling, printing, clamping and so forth. The devices can also be used to position individual parts for automatic assembly, etc. In each of these jobs, repetitive, precise and accurate movement in the face of undesired external loads is essential.
Pneumatic and hydraulic operated fluid devices accomplish movement of a tool or work piece by a power mechanism acting on a tooling plate. One conventional power mechanism includes a double action piston located within a cylinder and integrally connected to a piston rod. Pneumatic or hydraulic pressure is applied to either side of the piston so that a pressure differential is created across the piston. The differential pressure in the cylinder controls the location of the piston. It causes the piston to displace within the cylinder until the force on both sides of the piston is equal. The displacement, or stroke, of the piston rod is generally limited to the distance the piston can displace within the cylinder. This type of a system can be disadvantageous if the fluid medium is compressed air and the piston is floating in the cylinder and finally positioned by equal fluid forces being established on opposite sides of the piston. In heavy machine tool work, the forces created between the tools and the work can add to the force on one side of the piston within the cylinder, upsetting the equilibrium and throwing the tool out of alignment.
One manner of overcoming this disadvantage has been to utilize a plurality of fluid-actuated cylinders, such as hydraulic cylinders that do not rely on the establishing of equilibrium pressure. These cylinders have piston strokes of varying lengths and are stacked in an end-to-end relationship to provide a more rigid connection between the controlled tool and the positioning device. Such a device is disclosed in U.S. Pat. No. 3,633,465 to Puster. The actuated pistons disclosed in Puster slide the cylinders a distance that is equal to the sum of the stroke lengths of each actuated cylinder. Sizing the cylinders so that each has a different stroke length allows the device to achieve a large number of positions. Conventional multi-stroke, actuated cylinders are not laterally stable and occupy an excessive amount of space during use. In addition, many of these conventional actuators utilize position feedback mechanisms for insuring the accuracy of the positioning of the tooling plate. Typically, these feedback mechanisms include sensitive electrical feedback loops that can cause radio frequency interference with the power and fluid control mechanisms. Also, the use of electrical feedback or position control mechanisms can require shaft encoders that impose a risk of sparks or shorts, thereby creating explosive or otherwise hazardous conditions.
It is an object of the present invention to overcome the disadvantages of the prior art. It is also an object of the present invention to provide a multi-stroke cylinder capable of accurately achieving a large variety of positions without the use of a position feedback mechanism.